1. Field of Invention
This invention relates to a non-contact, optical apparatus for rapid and precise scanning of the three-dimensional (3-D) occlusal profile of a dental cast, and especially to a non-contact, optical system and method for measuring the 3-D occlusal profile of a dental cast as required by modern crown reconstruction works. According to this invention, a structured light beam (a linear light pattern produced by projecting a semiconductor laser through an optical lens to serve as measuring means) is implemented in a probe body and projected onto an object to be measured at a desired orientation; plural capturing units are implemented to capture comprehensive information representing the 3-D occlusal profile, based on which information the 3-D occlusal profile is calculated through the triangulation principle, thereby allowing efficient and precise measurement of the 3-D occlusal profile of the plaster dental cast.
2. Description of Prior Art
The recent development of tooth restoration works has turned progressed from the conventional tooth mold modeling and precision de-waxing casting techniques onto 3-D occlusal profile measuring technique in cooperation with dental cast reconstructions by computer-aided-design (CAD), as well as dental cast milling by computer-aided-manufacturing (CAM). The manufacture of artificial teeth, in responsive of the current market demands, focuses on the rapid and precise procedure in the hope of minimizing each patient's dental visits to complete the course of tooth inlay treatment.
The 3-D occlusal profile measuring technique nowadays mainly adopts the following two approaches, including: direct measuring within the mouth cavity, and plaster dental cast scanning, involving respective advantages. One of the advantages of direct measurement within the mouth cavity is its high speed with minimum material consumption. However, when precise measurement of 3-D occlusal profile is required to be conducted within a narrow space defined by the mouth cavity, the measurement is greatly affected by the limited space and complex surroundings, such as the saliva secretion of the patient and residual treatment materials, which confine the measurable range to only acquire partial occlusal profile in each scanning operation, and easily causes data errors due to measurement occlusion; meanwhile, the costly price for commercially available measurement system is the major difficulty in promoting such system that accounts for only 1.5% of the current market share. Though various measuring techniques have been developed in the past few years to accommodate the plaster dental cast scanning technique, there still is a need for the research and development of professional plaster dental cast scanning system, which features with integrated scanning functions to provide the 3-D occlusal profile of a full-dental cast in a rapid and precise manner and is capable of generating data fully compatible with conventional CAD/CAM system at a cost that is acceptable by most dental clinics.
The so-called non-contact 3-D occlusal profile measuring technique implies the use of various approaches to generate a light to be projected onto an object surface for acquiring the information characterizing the object surface through appropriate optical path principles and optical sensors, which could be subdivided into an active type and a passive type in accordance with the way that the light source is projected. The active type non-contact 3-D occlusal profile measuring technique is realized by projecting desired structured light patterns, such as a light spot array, sinusoidal periodic waves, optical beams or other meaningful light patterns, toward the object to be measured. Since the changes in the profile curvature or depth along the object surface will deform the structured light patterns projected onto the object surface, it is necessary to acquire the image of said deformed patterns by adopting appropriate measures, and then to reconstruct the 3-D profile data according to the acquired information, through the triangulation or phase shift principle. On the other hand, the passive type non-contact 3-D profile measuring technique is realized by acquiring the information characterizing the object surface under natural lighting through the optical image technique.
The research of non-contact high-speed acquisition of 3-D profile of objects has been widely investigated. However, there is little research literature aiming at the scanning of occlusal profile. Introductions and discussions with respect to the presently available 3-D occlusal profile measurement system, the measuring principles as applied, as well as its limitations as provided as follows to analyze the underlying principles and limitation in applications of the major commercially available 3-D occlusal profile scanners:
(1) Holographic imaging developed by B. Altschuler (1975):
In this system, laser holography is employed to acquire the 3-D occlusal profile and dimensions of a plaster dental cast, with a measurement resolution reaching 10 microns. This system uses two laser sources, each generating dot and line patterns (Raster patterns). Two raster patterns overlapping each other in a perpendicular manner cause interference. The 3-D data of the object are thus acquired from the changes in the phase of the interferences. However, the high cost of the equipment prevents from subsequent development and actual commercialization.
(2) The Duret/Hennson/Sopha system (1988):
Durent's method is based on the principle of laser holography, which is similar to Altschuler's method. Durent's method primarily made improvements in performing separate regional measurement at various viewing angles aiming at occlusion regions and acquiring 3-D dental cast data by overlapping the 3-D images.
(3) Method Rekow/Erdman-“Minnesota” system (1988):
Developed by Dr. Rekow in the University of Minnesota by utilizing stereo cameras to directly measure the 3-D occlusal profile from the mouth cavity, the primary techniques involved in this method reside in the precise calculation of the 3-D occlusal profile and dimensions based the acquired two dimensional high-resolution image sets. Meanwhile, another character of this system is to develop one occlusal profile database to support the establishment of a full-occlusal profile by computer aided design (CAD) thereby ensuring complete construction of the 3-D occlusal profile. However, since there still remain many problems to be resolved by the 3D stereo detection with respect to the precise measurements of occlusion regions, this system is not presently practicable.
(4) The Procera system (1994):
This system utilizes a mechanical, analog probe to continuously perform contact type measurements of the 3-D occlusal profile and dimensions of a plaster dental cast. The major advantage of said system is the high digitizing accuracy which is within 1-2 microns; however, the contact type measurement resulting in measuring efficiency that is far less than other non-contact type methods, and becomes a major limitation for such system.
(5) The Microdenta system (1995):
This method performs 3-D scanning of a plaster dental cast by utilizing the 3-D linear laser scanning method and an X-Y positioning platform. Such a method significantly improves efficiency of measurements than as to a contact type probe; however, the digitizing accuracy of the measurements is easily influenced by many factors, such as occlusion regions and measurement surfaces due to the congenital limitation of laser measurement, which could not guarantee the general accuracy of the measurements to be within a certain range.
(6) The Computer-Aided Prosthetic system, CAP (1991):
This system utilizes a single-point laser probe to perform the 3-D measurements of a plaster dental cast in cooperation with a two-axis dental cast rotating positioner. The advantages of said system reside in the enhanced digitizing accuracy of the measurements because of the scanning flexibility provided by the single-point laser as well as the positioner, to eliminate the occurrences of the occlusion regions during the measurements; however, the drawback and the primary limitation for such system is slow measurement that takes about thirty minutes in comparison with the use of a linear laser.
(7) The Showa/Nissan CAD/ACM system (1995):
This system performs 3-D measurements at different viewing angles by utilizing a Laser probe and a two-axis dental cast positioner, which significantly enhance the measuring efficiency. Another advantage of this system is the use of a common chamber for cutting of the mouthpiece and the measurement of the dental cast to effectively reduce the equipment cost of the system.
(8) The CEREC Method (1996):
This system is widely used in today's market. Along with the system development, two major ways have been developed for the 3-D measurements of an occlusal profile for ceramic reconstructions, which are (1) plaster dental cast scanning, and (2) intra-oral type dental cast scanning:
(8.1) The plaster dental cast scanning system: the principle involved in taking measurements for this system is very similar to the CAP system, with the measuring time at about thirty minutes for every two teeth and the range of measurements is within the width of two teeth, so as to increase the time for full-range dental cast measurements considerably and becomes a limitation in applications.
(8.2) CEREC system (disclosed in U.S. Pat. No. 6,263,234) is one of the few measurement systems that could be used to perform 3-D dental scanning inside the mouth cavity. This system projects a grid structured light of infrared (an invisible light) onto the dental occlusal profile inside the cavity. The camera of the system captures images every ¼ interval within four grid movements to acquire the relationship between the depth of the object to be measured and the amount of deformation of the grid pattern (in parallel stripes) to calculate the phase shifting differences for calculation of 3-D dimensions. Since the projected stripes of this system are fixed, the digitizing accuracy and the range of measurement may not be unsatisfactory due to difficulties in measuring occlusal profile with steep stepped surfaces.
The following conclusions could be reached from of detailed investigation of the above-mentioned scanning systems and principles of measurement. That is, in terms of the degree of digitizing accuracy and degree of reliability, the active light used in a non-contact type measurement is more feasible as compared to other means such as passive detection methods. In term of the scanning digitizing accuracy, the measuring principle and strategy adopted in measuring the occluded surfaces (generally directed to the regions that cannot be easily measured) of the occlusal profile are the key factors for acquiring precise measurements. Although many types of 3-D laser measurement probes have been commercialized in the market in recent years, which are widely used in reverse engineering, such measurement system could not provide direct and effective measuring functions for the particular and complex free surfaces of 3-D occlusal profiles.